This invention pertains to the field of metal forming and, more particularly, to the forming of materials which exhibit superplastic characteristics.
Super plasticity is the characteristic demonstrated by certain metals which exhibit extremly high plasticity in that they develop unusually high tensile elongations with minimum necking when deformed within limited temperature and strain rate ranges. The methods used to form the superplastic materials capitalize on these characteristics and typically employ gas pressure to deform sheet material into or against a configurational die to form the part. Diffusion bonding is sometimes associated with the process. Many U.S. Patents have issued which relate to the process itself, e.g., U.S. Pat. No. 3,340,101 to D. S. Fields, Jr., et al., U.S. Pat. No. 4,117,970 to Hamilton, et al., and U.S. Pat. No. 4,233,829, also to Hamilton, et al. Other processes combine diffusion bonding with the superplastic forming to produce much more complex structures such as U.S. Pat. No. 4,217,397 to Hayase, et al. to produce sandwich structures. All of these references teach a process which attempts to control stress in that they control the Pressure against the sheet being deformed versus time. One known exception to this rule is U.S. Pat. No. 4,489,579 to J. P. Daime, et al., which will be discussed infra. Furthermore, when the process is controlled by pressure versus time, there is no positive way of knowing where the specimen is at any given time.
The classic equation which defines the relationship between the variables in superplastic forming is: EQU .delta.=K.epsilon..sup.m
where m is the strain rate sensitivity, .delta. is stress, .epsilon. is strain rate, and K is a constant.
In the absence of strain hardening, the higher the value of m, the higher the tensile elongation. Solving the classic equation for m, ##EQU1## In addition to strain rate, the value of m is also a function of temperature and microstructure of the material. The uniformity of the thinning under biaxial stress conditions also correlates with the value of m. For maximum stable deformation, superplastic forming is optimally performed at or near the strain rate that produces the maximum allowable strain rate sensitivity. However, because the strain rate sensitivity, m, varies with temperature as well as strain rate and microstructure, which in turn varies with temperature, time and strain, m is, as a practical matter, constantly varying during the process. This is borne out by the fact that rather low forming stresses may produce the entire deformation if applied for a sufficient amount of time. However, significantly less time is required at increased forming stresses
Furthermore, it should be reasonably obvious that the strain rate varies at different instants on different portions of the formation inasmuch as stress levels are non uniform. The more complex the part, the more variation there is, and, therefore, strain rate differs over the various elements of the formation. Since strain rate sensitivity, strain rate, stress and microstructure are all inter-dependent and varying during the process, the relationship is theoretical. As a practical matter, there is no predictable relationship which can be controlled so as to form all portions of complex parts at the best strain rate sensitivity and the best strain rates. However, the artisan can plot strain rate sensitivity (m) against strain rate (.epsilon.) and stress (.sigma.) against strain rate (.epsilon.) and establish the optimum ranges to be used as guides. Those skilled in the art must then select and control those portions of the formation which are more critical to successful forming while maintaining all other portions at the best or less than the best strain rates which necessarily becomes the overall optimum rate. Excessive strain rates cause rupture and must be avoided.
As indicated supra, a single reference that teaches other than controlling the process by controlling pressure versus time is Daime, et al. This reference teaches a device for monitoring the forming steps by providing a tube which penetrates the die and engages a portion of the blank to be formed. As the blank is formed, the tube advances through the die directly as that portion of the blank is formed. Means are provided to provide a signal at predetermined amounts of advancement of the tube. The reference further teaches electrical contacts at recess angles of the die which are closed when the blank arrives at the electrical contact. Daime, et al. does not teach measurement of total strain or total deformation of the blank at any given time in the process or the volume of the space displaced by the deformed blank at any given time in the process. It teaches only the measurement of the deformation of the portion of the blank contacting the tube.
It is an object of the present invention to provide an apparatus and method for controlling superplastic forming processes by measuring and monitoring the incremental volume displaced by the blank being formed or total strain, i.e., surface area increase of the blank.